Delaying the start of childbearing raises the issue of fertility
postponed versus fertility foregone. One of the limits of previous
studies of ``How late can you wait?'' is the difficulty of controlling
for sexual activity. Data on the frequency and timing of intercourse
within a menstrual cycle are uncommon. We use such data from the
Menstrual Cycle Fecundability Study to study ``Can we wait until the
early 30s?''. We model the effect of age on conditional fecundability,
i.e., the probability of conception given that the couple is not
sterile, simultaneously controlling for the effect of primary sterility
and the frequency and timing of intercourse in each menstrual cycle.
Can we wait until the early 30s for a first birth? Our evidence is yes
(providing you are not already sterile) as the increase in the mean
waiting time to conception is very modest and of little practical
importance.